Basically, pressure sensitive copying paper consists of two sheets of paper, the lower surface of the upper leaf having applied thereto minute capsules containing a color reactive achromatic substance (hereinafter called a "color former") dissolved in a low volatile solvent and the upper surface of the lower leaf being coated with a solid acid, such as acid clay, zeolite, bentonite, attapulgite & phenolic compounds, etc. When the lower surface of the upper leaf and the upper surface of the lower leaf are put together and pressure is applied to the upper surface of the upper leaf by handwriting or typing, the capsules under pressure are broken and the color former contained therein is adsorbed by the solid acid on the upper surface of the lower leaf to develop a color, thus providing a copy.
There is also a type of pressure sensitive copying paper having a mechanism to develop a color when a regional pressure is exerted upon a sheet of paper holding capsules, which contain a color former, and a solid acid on or within it. However, such type is quite similar to the first-mentioned pressure sensitive copying paper (consisting of two sheets of paper, an upper and lower leaf) with respect to the mechanism of developing a color, and thus the same color former can be used for both types. And any of the above-mentioned facts belongs to the prior-known arts.
Conventionally, a number of compounds have been known to be useful as the color former, such as lactones, i.e. crystal violet lactone and malachite green lactone; thiazine compounds, i.e. benzoyl leucomethylene blue; and other leucoauramine and spiropyran compounds, and others. They have made it possible to manufacture various kinds of products which develop such colors as ranging from yellow and red to blue, green and black. However, the light resistance after development of color, particularly when inorganic substances such as acid clay, Silton (manufactured and soled by Mizusawa Chemical Company Ltd.), etc. are used as the solid acid, has been unsatisfactory, so that it has been hoped for to find out some solution for this defect. It is only benzoyl leucomethylene blue that shows a satisfactory light resistance, but it develops a color at a very low speed. Therefore it can not be used for practical purposes, unless it is mixed with some other color former which develops a color at a higher speed. Even this mixed use offers much inconvenience from a practical point of view, since benzoyl leucomethylene blue, which develops a blue color, imposes a restriction on the variety.